At present, the energy sector is responsible for about three-fourths of the anthropogenic carbon dioxide emissions. Over the past 15 years, economic activity in the U.S. has increased by about 50%; total U.S. electricity demand has increased 30% over the same period. In the coming years, the surge in the U.S. demand for electric power shows no signs of abating. Economic activity in the U.S. is projected to expand 49% by 2020. Accordingly, in the same period, the demand for electricity is projected to increase by another 30%. With this increase in electricity demand, CO2 emissions from more and more power plants will become an even greater driving force in rising atmospheric temperatures.
In general, there are three ways to reduce CO2 emissions from coal-fired power plants: (a) increase overall plant efficiency, (b) co-fire biomass, or (c) capture the carbon released by either pre-, in-situ or post-combustion processes. Due to the nature of human activities, there are double peaks and valleys for electricity consumption daily. Typically, the peak demand will be beyond the base-load power generation capacity and is met by quick-start up, natural gas, simple cycle turbine capacity. The valley demand will be below the base-load and is achieved by unit partial power output. Unfortunately, there is significant efficiency decrease in both period peak and valley demand. Assuming an average base-load plant efficiency of 40% in U.S. fossil fuel-fired generation fleet, one point of plant efficiency change will result in approximately 2.5% CO2 emission increase or decrease at the same gross electricity output. Stated another way, one point of drop in plant efficiency would result in approximately 4 million more tons of CO2 being emitted over the 40 year lifetime of 1 GWe of coal-fired power generation.
Capturing and storing carbon dioxide could slow down climate change and also allow fossil fuels to be a bridge to a clean, renewable energy future. Since the CO2 emitted from electric utilities is the present concern, faster implementation of CO2 capture by chemical means at stationary combustion sources would be highly desirable. While absorption/stripping with aqueous based (such as amine-based solvents) scrubber systems has been successfully used for natural-gas purification, it poses several technical challenges, including the fact that flue gas from utility boilers is at near atmospheric pressure and the concentration of CO2 in the flue gas is relatively low at 12-14%. Another technical hurdle is the energy requirements for the CO2 capture/desorption devices to regenerate absorber reagents. Generally speaking, the energy required for CO2 capture and sequestration using monoethanolamine (MEA) is estimated to reduce a PC plant's output by about 30 percent, which equates to a very substantial 60-80% increase in the cost of electricity. The ability to store energy from a utility grid to allow storing electrical energy during off-peak times and releasing energy to plant for peak time carbon capture will be highly beneficial and will allow a substantial reduction in energy production costs.